1. Field of the Invention
This invention relates generally to indirect dental restorations. Specifically, the present invention relates to a system for fabrication of indirect restoratives utilizing polymer composites which may be performed during a single dental office visit or in the dental lab by a technician.
2. Brief Description of Related Technology
For centuries, restorative dentistry has been performed utilizing the lost wax technique wherein gypsum dies created from impressions are made of the prepared teeth. Gypsum-containing setting materials require a certain amount of time to set up. Setting time of the restoration materials can be decreased, but at the peril of accuracy problems. Gypsum models also require mounting on articulators, utilizing plaster, requiring a certain amount of setting time to mount a model. Thus, regardless of the impression material used, the setting time required by gypsum-containing materials requires the dentist to make a second appointment with a patient to affix a permanent restoration.
An other problem with using gypsum-containing dies is that certain restorative materials tend to adhere to the gypsum, regardless of the separator material applied to the dies One of the reasons for this is that although separator materials may be prepared which work for most materials, it is undesirable to coat margin areas on the die with the separator material for fear of creating a space in this area in the finished restoration. Thus, technicians have expected to have die models chipped at the margins during fabrication This dilemma has been solved by duplicating the master model for fabrication purposes, necessitating an additional step which may create more error in material shrinkage/expansion parameters. Each time a master model is duplicated, accuracy is at risk. Another problem with duplicating the master model is that the master model must still be used to fit the restoration, creating the risk of wearing and chipping of the master model. Regardless of these problems, this technique, which requires at least two patient office visits, is still being utilized in the dental industry.
One alternative to gypsum dies has been to utilize a light-curable polymer composite inlay fabricated in the patient""s mouth, which is then removed from the mouth for final curing and then cemented in the patient""s mouth, during a single office visit. Problems with such a system include the fact that the fabrication field is not always free of saliva and blood, which can interfere with the setting of the composite. Another problem with such a system is that the occlusion may be overly high because the patient is not able to bite into the setting composite to form a functionally generated path-type restoration. Contacts are also a problem with such a system. The contacts must sometimes be added after removal of the composite from the mouth because, for example, the stainless steel band on a retainer may interfere and create an open interproximal contact.
Furthermore, even if the fabrication field is dry (free of blood and saliva), the contact adequate, and the occlusion easy to manipulate, the composite utilized in such a system may have a tendency to lock into slight undercuts in a prepared tooth. If the light-curable composite set inlay hardens in such tooth undercuts, the restoration cannot be removed for final curing. Also, since the composite in the proximal box area is tacky and soft because light cannot reach it, removal of the restoration may result in breakage or deformation of the composite pattern, particularly in large, complex restorations. One way in which this problem has been solved is to place the tip of a paper clip in the central pit area of the restoration so that the clip sticks like a sprue pin in a wax pattern, and thus provides a way to vertically remove the restoration.
Further problems with such a system include the formation of an overly dry surface after light cure outside of the mouth, inadequate cementation of the finished restoration onto the dentin and enamel bonded prepared tooth, and the previous lack of chair-side sand blasting technology,
The desire for chair-side availability of restorations also has led to the development of milling machines. An early machine included a copy-key device. The system included taking a pre-operative impression, preparing the tooth, and taking a post-operative impression with a stiff material. A stylus of the milling machine followed the contour of the intaglio surface of the post-operative impression, which was transferred to a milling arm, which cut the shape into a block of feldspathic porcelain. The occlusion was cut into the opposite side of the restoration by following the occlusal morphology of the poured pre-operative model with the stylus. Problems with this system included the difficulty in keeping the impression immobile on a pedestal while the stylus tracked over the surface of the impression. Furthermore, the cost of the milling machine prohibited many dental offices from utilizing this system.
Improved milling machines utilizing CAD/CAM technology are available, but are considered cost-prohibitive by many. Also, polishing is required for any porcelain restoration. A further draw-back is the monochromaticity of the porcelain.
The monochromaticity problem may be solved by replacing porcelain restoratives with those made from a variety of polymer composites. A current trend is to prepare permanent dental restorations from composite materials made by free-radical polymerization of methacrylate functionalized monomers or oligomers. The formed highly viscous polymerization products typically are cured by light, (e.g., by a visible light source) or by heat, or by a xe2x80x9cdualxe2x80x9d light- and self-cure. It also is known that traditional light-cure, self-cure and xe2x80x9cdualxe2x80x9d light/self cure composites may be further polymerized by exposure to heat.
In some instances, the presence of oxygen has been found to inhibit the light curing of free-radical initiated chemical systems. This oxygen inhibition is noticed on a cured surface as occlusal haziness or tackiness. To avoid these aesthetic flaws, some processes, such as the one disclosed in Yarovesky et al., U.S. Pat. No 5,000,687 teach light cure under an inert nitrogen atmosphere. Dual cure (i.e., light/heat) has been found to impart good post-cure strength to the composite product.
Conventional processes for fabricating restorative dental implants have proven quite expensive, and time consuming. Currently, dentists must send single and multiple unit restorations to off-site dental laboratories. These laboratories are the only source capable of manufacturing dental implants having strength, color, and other aesthetic qualities demanded by patients and their dentists.
Currently, the overall process to provide a patient with a final, permanent restoration requires the patient to make at least two separate dental visits. In the first visit, the dentist typically prepares a subject tooth, prepares and cements a temporary restoration, removes excess cement, checks for temporary occlusion, and prepares a final impression which is then sent to a dental laboratory where a permanent restoration is made. In the second visit, the dentist removes the temporary restoration, removes residual temporary cement, trial-fits the lab-fabricated permanent restoration, checks margins and occlusal harmony with opposing teeth, and then cements the permanent restoration.
There are numerous disadvantages of the current multiple-visit procedure. One notable disadvantage is the inconvenience to the patient in having to undergo an interim procedure where a temporary restoration is affixed and, subsequently, undergoing another procedure to remove the temporary restoration and then to affix the permanent restoration. Each visit may require anesthetizing. Furthermore, each visit requires the patient to take time away from work or home. Another disadvantage of the current multiple-visit procedure is the added cost to the dentist, such as the cost in foregoing the opportunity to attend to another patient, and in the added costs in preparing a dental operatory. Often times the preparations required for the added visits are not billable. The current multiple-visit procedures are also uneconomical, especially considering that many dentists"" offices, such as those associated with managed care programs, are staffed with qualified dental assistants who could be trained to fabricate permanent restorations having superior integrity and aesthetic qualities.
A known process for manufacturing a permanent dental restoration which can be completed in a dentist""s office during one patient visit, includes the use of a polyvinyl silicone (PVS) die in lieu of the traditional gypsum die. Instead of pouring a post-operative impression with gypsum stone, PVS having a high filler content is utilized. Such a PVS material does not stick to either alginate or hydrocolloid and also does not distort the material in its set. PVS impressions may be injected with this material if a silicone mold release agent is first sprayed into the post-operative impression In addition to excellent marginal accuracy, dies made from PVS materials allow for easy composite removal after polymerization of the composite restorative.
A variety of composites, including self-cure, light cure and dual cure (light/self cure) may be packed into these dies and cured at high oven temperatures without degradation of the PVS material. The composite material may be added in layers, fully curing each layer by light in an oxygen environment prior to the addition of a subsequent layer, and optionally, final curing the composite in an oven for a few minutes at about 250xc2x0 F. (121xc2x0 C.). Oven temperatures of up to 270xc2x0 F. do not affect the integrity of the die material thereby allowing multiple restorations to be done on the same die. Advantages of such a system include the fact that shrinkage of the restorative occurs outside of the mouth and occlusion can be ground on the bench rather than in the mouth.
However, formed restorations made according to such a procedure have lacked the strength, integrity and aesthetic qualities characteristic of the restorations made in off-site dental laboratories. Also, the progressive loading of material requires that additional cosmetic material be added over the formed restoration to increase its occlusal loading. The occlusal loading is necessary to minimize the effects of haziness thought to be formed by light curing in the presence of oxygen. Another drawback is the oxygen inhibited layer that forms during polymerization, resulting in a tacky relatively soft surface of the restorative.
Thus, it remains desirable to provide a system for preparing dental restoratives of improved strength and aesthetic qualities which can be made in a dentist""s office during a single patient visit without the aid of the highly skilled technicians of dental laboratories.
It is an object of the invention to overcome one or more of the problems described above.
In a process according to the invention, a first, pre-operative impression of a tooth to be restored is taken using a first polyvinyl silicone (PVS) material disposed in a clear tray, followed by the preparation of the tooth. A second, final post-operative impression of the tooth is taken utilizing a second PVS material. A tooth model is then made by pouring a low viscosity and suitably rigid, third PVS material into the final impression. Prior to the complete setting of the low viscosity PVS material, a higher viscosity, fourth PVS material is applied onto the model PVS material to form a completed model with base. The model is then fitted into the pre-operative impression matrix made from the first PVS material. Layers of composite are packed onto the impression in the clear matrix. The model (with attached base) is then inserted into the composite disposed in the first PVS material and the resulting matrix is placed in a single apparatus for conducting both light and heat cure of the dental composite.
Also according to the invention are curing apparatus and methods utilizing the apparatus to prepare a dental restorative. The apparatus can utilize a thermister disposed in a composite material, the thermister/composite assembly being disposed in a curing chamber of the apparatus and connected to a control device which controls a heat lamp also disposed in the apparatus. Curing of the dental composite is performed by heating until, for example, the thermister reaches a selected temperature.
Other objects and advantages of the invention may become apparent to those skilled in the art from a review of the following detailed description, taken in conjunction with the drawings and appended claims.